Multi-group transmissions consist of two or more transmission groups, usually arranged in series, by combining which a large number of gears can be produced. Increasingly, they are designed as automated gearshift transmissions consisting, for example of an input group, a main group and a downstream range group. Such transmissions are used in particular in utility vehicles since they provide an especially fine gradation of gears, for example with 12 or 16 gears, and are highly efficient. For a smaller number of gears configurations with only a main group and an input group or a main group and a range group are also possible. Furthermore, compared with manual gearshift transmissions they are characterized by high operating comfort and, compared with automatic transmissions, their production and operating costs are particularly economical.
By virtue of their structure conventional multi-group gearshift transmissions, like all manual or automated gearshift transmissions not shifted under load, undergo a traction force interruption during gearshifts since the force flow from the drive motor is always interrupted by disengaging a clutch in order to disengage the engaged gear without load, to synchronize the transmission and the drive motor in a neutral position to a connection speed, and then to engage the target gear. Since the vehicle is rolling during the traction force interruption undesired speed increases or speed decreases can occur. In addition the fuel consumption can increase. Whereas with passenger motor vehicles the traction force interruption, which affects the driving dynamics, is as a rule perceived only as annoying, for example during upshifts in a driving style of sporty orientation, in the case of medium-weight or heavy utility vehicles the driving speed can be reduced to the point where an upshift is made impossible and, on uphill stretches, undesired downshifts, creep-driving or even additional starting operations may be necessary.
From DE 10 2006 024 370 A1 by the present applicant a traction-force-supported automated multi-group transmission is known and has a splitter group as its input transmission, a main group as its basic transmission and a range group as its output or downstream transmission. The structure of this known multi-group transmission with its input group and the main group enables a direct gear to be engaged as an intermediate gear during a gear change. For this, a direct connection is temporarily formed between an input shaft of the input transmission and a main shaft of the main transmission by means of a change-under-load clutch. This renders the main transmission and the splitter group free from load, so that the engaged gear can be disengaged, the transmission synchronized and the target gear engaged. The change-under-load clutch transmits the motor torque to the transmission output, and a dynamic torque that is released during a speed reduction between the original and target gears is used to compensate for the traction force interruption to a large extent. A conventional, separate starting clutch, which can remain engaged during the gearshift operation, maintains the force flow between the drive motor and the input shaft. The change-under-load clutch can be positioned between the input group and the main transmission or between the starting clutch and the input group.
Moreover, from DE 195 48 622 C1 a countershaft gearshift transmission is known, in which a frictional clutch that can be driven by the drive motor, a claw clutch and a synchronous clutch are arranged between a drive motor and a transmission input. The input element of the frictional clutch can be connected by the claw clutch to a transmission input shaft. On the transmission input shaft are mounted fixed wheels of the 1st and 4th gears, which mesh with associated loose wheels on a parallel countershaft. The loose wheels can be connected in a rotationally fixed manner to the countershaft by synchronous clutches. For a 5th gear, conversely, the loose wheel is mounted on the transmission input shaft and the associated fixed wheel on the countershaft.
The output element of the friction clutch can be connected selectively by the synchronous clutch either to the transmission input shaft or to the loose wheel of the 5th gear. In a starting operation the synchronous clutch connects the frictional clutch to the transmission input shaft, from which the torque of the drive motor is transmitted via the gearset of the engaged 1st gear to the drive output of the countershaft.
After starting, the frictional clutch is disengaged and the claw clutch is engaged, i.e. the torque flow changes from the frictional clutch to the claw clutch. For a subsequent gearshift from 1st gear to 2nd gear the synchronous clutch is connected to the loose wheel of the 5th (the highest) gear and the frictional clutch is engaged. This removes load from the gearset of the 1st gear, whereupon the connection of the loose wheel of the 1st gear to the countershaft is released and the loose wheel of the 2nd gear is connected to the countershaft. The frictional clutch is then disengaged again, so that the torque flow is now passed on in 2nd gear via the claw clutch.
This known transmission enables gearshifts to be carried out without interruption of the traction force, and therefore performs the task it is intended for, especially in the starting range when changing from 1st gear to 2nd gear, namely to avoid torque interruptions. However, it is limited to five gears. But multi-group transmissions with 12 or more gears require further measures, not described in the document, in order to enable traction-force-supported shift sequences of the transmission groups.